Bending presses are used to form sheet materials, and for that purpose they comprise one or more tools commonly referred to as “punches” fixed on the moving panel of the machine by means of a tool carrier or fixing system, and facing the tools, bending presses comprise one or more V-shaped dies that are fixed to the top edge of the bottom panel of the machine.
By placing the sheet that is to be formed on the die and lowering the punch through a predetermined distance, the sheet is bent through the desired angle.
In addition, depending on the type of forming and on the dimension of the forming to be implemented on the sheet materials, different tools are used. It is therefore necessary to be able to dismount the tool from the top panel, i.e. from the fixing system, so as to replace it with some other tool that is more suitable.
As is well known, the tool includes a fixing shank and it is fixed by means of a pivotally-mounted clamp capable of occupying a first position in which it clamps the tool or a second position which is spaced apart therefrom and allows the tool to be changed.
It should also be understood that bending tools are relatively heavy. It is therefore very desirable to hold the tool against the clamp even when it is in its loose position so as to avoid any danger of the tool dropping onto the die of the bottom panel placed facing the tool.
In French patent application FR 00/07415 in the name of the Applicant, an embodiment of a tool carrier or tool-fixing system is described that enables pivoting of the clamp to be controlled effectively and that also enables the tool to be retained after the clamp has been loosened.
Accompanying FIGS. 1A and 1B show the embodiment of the tool-fixing system described in the above-identified patent application. These figures show the moving top panel 10 and the tool spacer 12 fixed by any appropriate means to the top panel 10. The tool spacer has a bottom portion 14 of reduced thickness, defined by two shoulders 16 and 18.
The fixing system also has a clamp 20 mounted to pivot about a horizontal axis parallel to the long direction of the panel. The pivot axis may be defined by rocker-headed screws 22 co-operating with corresponding holes 24 machined in the middle portion 26 of the clamp 20. To pivot the clamp 20 about the axis defined by the heads of the rocker-headed screws 22, it is possible to use pushers 28 each having one end acting on the top end 30 of the clamp 20 and a second end co-operating with a rotary cam 32 mounted in the tool spacer 12 and extending in the longitudinal direction of the panel. In that embodiment, when the cam 32 is in the position shown in FIG. 1A, it causes the top portion 30 of the clamp to move away, which corresponds to the clamping position. In contrast, when the cam 32 is in its position shown in FIG. 1B, it no longer acts against the pushers 28, and under the effect of a resilient system, the clamp 20 takes up the mounting and dismounting position for the tool as shown in FIG. 1B.
In its top portion, the tool 34 has a fixing shank 36 which presents a first fixing surface 36a, a second fixing surface 36b parallel to the first fixing surface, and a retaining groove 38 situated beneath the second fixing surface 36b and presenting top and bottom edges 38a and 38b. 
In the clamping position shown in FIG. 1A, the fixing surfaces 36a and 36b of the tool shank are clamped between the clamping surface 14a of the end 14 of the tool spacer and the clamping surface 40 of the bottom portion 42 of the clamp 20.
In addition, at its bottom end 42, the clamp includes retaining means in the form of a catch 44 capable of penetrating into the groove 38 of the shank of the tool 34. When the clamp 20 is in the clamping position as shown in FIG. 1A, the catch 44 does not perform any function. In contrast, when the bottom end 42 of the clamp is spaced apart to occupy the mounting and dismounting position, the catch 44 remains engaged in part in the groove 38, as shown in FIG. 1B, thereby enabling the tool 34 to be held in the vertical direction relative to the tool spacer 12.
That system enables the tool to be retained effectively and prevents it from falling, nevertheless it suffers from the following drawback. When the clamp 20 is brought into its mounting and dismounting position, the tool 34 can be removed only via one of the longitudinal ends of the clamp 20 by sliding the shank of the tool between the clamp and the bottom portion 14 of the tool spacer.
It will be understood that since the tool cannot be put into place between the bottom portion of the tool spacer and the clamp other than sideways, i.e. it can only be put into place via an end of the clamp, when the operator seeks to replace a tool which is mounted in the central portion of the clamp, for example, it is necessary for the operator to begin by removing all of the tools placed between the tool that actually needs to be changed and one of the ends of the clamp.
Such relatively difficult operations of removing a plurality of tools significantly increases the time required to change the desired tool, and thus reduces the productivity of the bending press. The greater the frequency with which tools need to be changed, the greater the loss of time.
Also known is a fixing system for a bending tool as described in U.S. Pat. No. 5,022,256 in the name of Machinefabriek Wila B. V. That system likewise comprises a tool spacer and a clamp mounted to pivot about a horizontal axis parallel to the long direction of the panel, with pivoting of the clamp being controlled by hydraulic actuators.
In addition, the system has an elastically-deformable metal strip mounted to pivot about a horizontal axis parallel to the long direction of the panel on the clamp and which presents a first surface facing the clamp and a second surface facing the bottom portion of the tool spacer. A rigid piece is fixed on the second surface, in the bottom portion of the metal strip. The rigid piece has a bottom portion presenting an inclined surface going towards the metal strip.
When the clamp is spaced apart to occupy the mounting and dismounting position for the tool, the metal strip is separated from the clamp and the top edge of the rigid piece co-operates with a shoulder formed on the fixing shank of the tool so as to prevent the tool from falling.
When the clamp is brought into the tool-clamping position, the fixing shank of the tool is clamped on one side by the bottom portion of the tool spacer and on the other by the rigid piece which transfers to the tool the force that is exerted thereon by the clamp.
The assembly constituted by the metal strip and by the rigid piece is thus subjected to very high compression stresses, thereby requiring materials that are capable of withstanding such stresses to be used for making these various parts and the fixing means for connecting the parts to one another.
In order to limit the stresses to which said assembly is subjected, U.S. Pat. No. 5,022,256 makes provision for placing a strip of flexible plastics material between the clamp and the metal strip, the plastics strip being housed in a groove formed in the clamp. That solution then presents the drawback of making the shape of the clamp more complex.
Furthermore, in that type of system, the elastically-deformable metal strip is subjected to compression stresses that are high and regularly repeated, causing it to lose its properties of elasticity progressively, which is harmful for proper operation of the system.